Cohesion, << koh HEE zhuhn, >> is the force that holds a material together. It results from the attraction that atoms and molecules have for one another. This attraction decreases as the distance between particles increases. Thus, with few exceptions, cohesion is highest in solids. Liquids are less cohesive than solids, and gases are practically noncohesive. Powders can also exhibit cohesion, especially if they consist of fine particles packed together. Packed dirt, for example, can be a solid driving surface because of its cohesive properties.
Because of cohesion, effort is required to separate a material into two parts. This effort is called the work of cohesion. The work required to separate the material is twice its surface tension because two new surfaces have been created (see Surface tension ).
Scientists can calculate the theoretical maximum stress a solid can withstand before breaking if they know the work of cohesion. This maximum is known as the solid’s tensile strength. These calculated strengths are usually much higher than measured values. Scientists believe this is so because small surface cracks and other imperfections develop in solids and make them easier to break. Glass fibers, for example, have great resistance to breakage when first manufactured, but they develop fine cracks and lose strength rapidly.
